Not all skills needed to be a good QA are technical. In the long run, the lack of so-called “soft” skills costs the team in general and QA engineer in particular more expensive than the lack of technical knowledges.
The main stakeholders for any functionality being developed are the business side (or product owner), UX designer, developer, and QA engineer. These 4 sides represent the point of view of what the system should do, how it should look, what can and can not be implemented, and what may go wrong or be missed.
Not the “best practice” like combining of several roles in the single one person could be very effective for a small team where extra team-member adding may leads to misunderstanding and loss of information.